Developmental Disorders of the cerebal hemispheres

E. Developmental disorders of the cerebral hemispheres (Many of these are associated with mental retardation)

1. Hydrocephalus - increased ventricular size due to increased quantities of CSF usually under increased pressure (see section III)

2. Polymicrogyria - increased number of small irregular gyri; not necessarily accompanied by mental retardation; commonly seen with the Arnold Chiari malformation.

3. Pachygyria - very broad thick gyri; often associated with mental retardation.

4. Hydranencephaly - cerebral hemispheres are replaced by thin-walled sacs filled with CSF; results from massive destruction after the 12th week of gestation from vascular infections or traumatic disorders. Transillumination of the skull occurs.

5. Heterotopias - clusters of neurons in abnormal locations, usually in white matter; generally result from a failure of migration of nerve cells from the subependymal region to the cortex. These abnormal foci of neurons may cause seizures.

6. Porencephaly - cyst formation in the brain with cerebral cysts that extend from the ventricular system to the subarachnoid space. These are generally caused by destructive, infectious or vascular lesions. A familial type has been described with dominant inheritance.

Example of porencephaly

7. Holoprosencephaly - condition in which the cerebral hemispheres fail to divide properly. Some cases are associated with trisomy 13-15 and other chromosomal defects. This condition involves total or partial lack of division of telencephalic vesicles, optic vesicles, and/or olfactory vesicles.

8. Megalencephaly - brain much larger than normal (greater than 1800g). This condition may or may not be associated with mental retardation.

The coronal section on the left us from a megalencephalic brain; compare to normal sized section on the right.

9. Microcephaly - brain smaller than normal (less than 900g in adult) without degenerative lesions; usually associated with mental retardation. Associated with many inherited syndromes.

10. Agenesis of the corpus callosum - may occur as an isolated anomaly without any clinical dysfunction, but this condition is often associated with other malformations.